The separation of paraxylene from the aromatic C.sub.8 cut is conventionally effected either by one of the crystallisation processes or by the procedure involving adsorption in a simulated mobile bed (see the prior art of French patent No 91/11004). The separation of orthoxylene is conventionally effected by superfractionating.
For those processes the aim to be achieved is the production of an isomer with a given level of purity in a manner which remains constant with the passage of time.
An excessively low level of purity can very seriously reduce its commercial value to such a degree as to make it impossible to sell while an excessively high level of purity gives rise to an increase in operating costs.
Hitherto it was possible to achieve that check on purity in two ways:
1) Measurements of flow rates and analysis operations at points in the flows passing into and issuing from the unit in such a way as to establish detailed balance sheets with respect to the materials involved. Then, by comparison with model material balance sheets and correlating the influence of each of the operational variables, possibly modifying one or more operational variables in order to stay within the desired range of values in regard to yield and purity. The frequency with which that kind of balance sheet is established is from one to several times per day. That means that in order to guarantee a predetermined level of purity in spite of the minor random fluctuations in the operational variables, it is necessary to aim at a level of purity which is slightly greater, which has an adverse effect on operating costs.
2) In-line analysis of the effluents by means of vapour phase chromatography. That method makes it possible to react quickly in the face of a drop in purity which suddenly occurs. For the product which is outside the specification may be recycled in order not to pollute the storage tank which is in the course of being filled. In addition, measurement of the operational variables and parameters will make it possible to rapidly modify the appropriate setting or settings. Such modification may be effected either by means of empirical correlations or by means of software for dynamic simulation of the unit which is based on predictive modelling of the unit. The article `Diagnosis process problems` in chem. Eng. Prog. V87, No 9, pages 70-74 (1991) summarizes the most advanced state of the art.
As regards more precise analysis of the level of purity of p-xylene, it can be measured by gaseous phase chromatography or by measurement of the crystallisation point. The method involving measurement of the crystallisation point only works with relatively rich mixtures (at least 8%) of paraxylene, and it is therefore not universal, even for paraxylene. It was also analysed by near infrared spectroscopy (Ed Stark et col. SPIE Vol 1575, pages 70-86, 1991). The optical fibers which can be used in near infrared have higher coefficients of attenuation than the used in the visible range and do not allow the use of very substantial transmission distances.
The frequency of analysis by means of vapour phase chromatography is such that it is however only possible to carry out a few measurements per hour and the result is produced only some ten minutes after the sampling operation.
In the case of recrystallisation however, provided that the composition of the charge to be treated is indeed constant, precise temperature measurements at different points in the process permit excellent regulation of the degree of purity of the paraxylene produced.
Unfortunately, in the case of chromatography in a simulated mobile bed, simply measuring a parameter such as temperature does not make it possible to establish a purity regulation chain.
In the case of distillation (production of orthoxylene by superfractionating) simply measuring temperature is not sufficiently precise since the total temperature difference or variation is from 5.degree. to 7.degree. Celsius for 300 real plates. We are therefore proposing measurement in real time simultaneously at a plurality of points which are internal to the separation process of the composition of the mixtures in a fluid phase, with a measurement apparatus which is disposed away from the separation unit. The analysis method selected is Raman spectroscopy.
The literature is very abundant as regards the potentialities of Raman spectroscopy for in-line real-time and multi-point analysis of solutions in an industrial environment. S M Angel et col, (SPIE Vol 1435, Optical methods for ultra sensitive detection and analysis, Techniques and Applications (1991)) describe a system made up of a laser diode, an FT-Raman spectrometer and a CCD detector (Charge Coupled Diode). The fiber length is 4 m. In order to attenuate the Raman spectrum of the collecting fiber, two emitting and collecting fiber configurations are proposed. The ends of the fibers are disposed at 360.degree. or 180.degree. from each other, The solutions analysed are pure solutions of naphthalene and toluene. No quantitative analysis is effected. The same authors in SPIE Vol 1587, Chemical, Biochemical and Environmental Fiber Sensor III (1991), use the sees assembly for simultaneous analysis at four points with four laser diodes. The emitting and collecting fibers are positioned in facing relationship or form an angle of 15.degree.. The acquisition times vary between 1 and 30 seconds. The assembly is used for monitoring a column for distillation. In the case of water/ethanol separation by distillation, the percentage of ethanol is determined after calibration. A potential application related to distillation of petroleum cuts is discussed with quantification of the percentage of toluene and benzene with acquisition times of 60 seconds.
M J Roberts et col. in Process Control and Quality, 1 (1991), 281-291, Elsevier Science Publishers B. V. Amsterdam, describe the limitations of Raman spectroscopy for in-line real-time and multipoint analysis, for the purposes of chemical analysis. Different factors such as the type of components, their number, their concentration, the number of analysis points, the level of precision and the response time are discussed. In order to be free of the Raman spectrum of the collecting fiber, calculations are made to determine the angle between the collecting fibers and the emitting fibers, taking account of the refractive indices of the medium and attenuation of the fibers in relation to the wavelength of use. The Raman spectrum of cyclohexane is present with 35 m of optical fibers of silica, positioned at an angle of 10.degree..
The same authors, in ISA, 1990, 0065-2814/90/pages 463-468, apply the following assembly to the operation of a water-isopropanol distillation column: the source is formed by a Ya g-Nd laser, while a mechanical multiplexer makes it possible alternatively to select one of the six emitting fibers. The emitting and receiving fibers are made up of fibers of 400 .mu.m in diameter, of doped silica-silica. The optical sensor in the true sense is formed by the assembly in a stainless steel tube terminated by a sapphire window of 1) in a central position, the emitting fiber, and 2) at a peripheral position, 4 to 6 collecting fibers. The measurement cell is formed by a cylindrical cell of a minimum dimension of 90 mm by 50 mm in diameter, equipped with a flowmeter and a thermocouple. The spectrometer is equipped with a filtering system permitting elimination of the Rayleigh diffusion signal, for each spectrum, the acquisition time is 2.2 minutes and the degree of precision of chemical analysis is 3% on mixtures in the range of 5%-95% to 95%-5%.
M A Leugers et al in SPIE, Vol 1990, Chemical, Biochemical and Environmental Applications of Fibers (1988), developed a probe formed by a collecting fiber at the centre and six receiving fibers around it. The probe is used for quantitative analysis of benzene/toluene mixtures. When applied in the petroleum field, Raman spectrometry is used (U.S. Pat. No 5,139,334) for the measurement of properties such as the octane number, the percentage of aromatics and mono/di-aromatics distribution. U.S. Pat. No 2,527,121 discusses determining the amount of total aromatics in mixtures of hydrocarbons by Raman spectrometry.
It is found therefore on the one hand that analysis of the aromatic cut C.sub.8 when pure or diluted by a solvent (mixtures of 4 or 5 constituents in any proportions) has never been effected with that type of assembly, and on the other hand quantification of mixtures, even binary mixtures, is still fairly imprecise and generally does not cover the whole range of composition. On the other hand, in the prior-art assemblies, the authors have recourse either to a plurality of laser sources or to mechanical multiplexing: these are example and expensive constructions. Concerning the use of a plurality of sources, when these are laser diodes, the spectral width is generally fairly substantial and it does not make it possible to obtain spectra which are appropriate for furnishing precise quantification of the mixtures having four or five constituents. When lasers are involved, besides the considerable increase in costs that this entails, it is difficult to provide that all sources emit precisely on the same wavelength, which is revealed by virtue of the fact that each of the spectra obtained is slightly displaced with respect to the others and that it is not possible to use a single quantification software. In addition, the optical sensors and the measuring cells proposed are also expensive end sophisticated. Finally, in the situation where optical fibers have been used, they never exceed a length of 50 m, which makes it entirely possible for the laser source and the spectrometry to be moved away to a control room or a laboratory. However moving the laser source and the spectrometer away in that fashion is absolutely essential since it is illusory to try to place sophisticated and non-deflagrationproof optical instruments directly within an industrial separation unit.